1. Field of the Invention
This invention relates to a polishing member and a polishing apparatus for polishing wafers and more particularly to a polishing member and a polishing apparatus which are adapted for the technique of planarization machining aimed at conferring improved flatness on semiconductor devices.
2. Description of the Prior Art
In consequence of the advance of the trend of semiconductor devices toward greater integration and larger capacity, the technique for imparting minimized diameters to wires and the technique for increasing the number of component layers of multilayer wires have been acquiring growing importance.
When a wire has a minimized diameter, the ability of an insulating film to be superposed on the wire or the ability of the wire to be covered with the insulating film is degraded because the end face of the wire inevitably gains in precipitousness. When a multilayer wire has an increased number of component layers, it betrays heavy surface irregularities because of accumulation of irregularities on the underlying layers or on the insulating film. When a wire is to be superposed on the surface of this multilayer, the superposition is attained only with inferior wiring precision because the stepper is no longer focussed accurately on the irregular wire surface. In any event, these surface irregularities tend to cause breakage in the wire and impair the reliability of a semiconductor device using the multilayer wire.
Various techniques for flattening wire surfaces have been developed for the purpose of solving this problem. The glass flow method, for example, aims to provide a wire with a flattened surface by forming a glass film such as of PSG, BPSG, etc. by the CVD and then heating the glass film at a temperature in the range of from 800.degree. to 1,100.degree. C. thereby generating viscous flow of the glass film. Though this method is simple as a process, it is at a disadvantage in limiting the material to be used for the wire because of the high temperature which is required for heating the glass film and, therefore, is not tolerated by aluminum. Various other methods have been developed. They have both merits and demerits. None of them perfectly fits the purpose of surface smoothing under consideration.
In recent years, the researches after a method for producing a smooth surface by utilizing the technique of wafer polishing has been under way with a view to overcoming this discouraging state of prior art. Specifically, in the process of manufacture of a semiconductor device, the researches are aimed at the application of the wafer polishing technique to the technique of planarization machining capable of exalting the flatness of the semiconductor device, namely the utilization of the technique as a measure to flatten the parts projecting from the surface of a silicon oxide film in conformity to the wires distributed on a wafer. Heretofore, this wafer polishing technique has been primarily intended to impart a uniform thickness to a wafer throughout the entire area thereof and, therefore, has been developed for the purpose of preferentially removing parts of an increased wall thickness from a wafer.
In the planarization machining technique for the manufacture of a semiconductor device, however, the necessity of developing the surface-based polishing technique, i.e. a technique which enables a wafer in the process of machining (hereinafter referred to as "wafer W"), even when the cross-sectional shape thereof happens to contain differences between parts of a large wall thickness and parts of a small wall thickness as shown in FIG. 7, to be so polished that the oxide film on the surface of the wafer W may be excoriated in an equal amount and the wafer W may assume such a cross-sectional shape as is illustrated in FIG. 8, has been finding widespread approval.
The reason for this necessity is that the wafer polishing technique has been heretofore developed for the purpose of preferentially removing parts of an increased wall thickness from a given wafer thereby attaining the impartation of a uniform wall thickness to the wafer throughout the entire area thereof. The surface-based polishing technique specifically consists in removing from a silicon substrate 31 illustrated in FIG. 7 protrusions 33 of oxide film, namely differences of level occurring in an oxide film 32 (interlayer dielectric) on the silicon substrate 31, and at the same time permitting the oxide film 32 to acquire a uniform thickness. In FIG. 7 and FIG. 8, 34 stands for an element and 35 for a wire distributed. In these diagrams, the global rises and falls in the wafer W are exaggerated for the sake of convenience of illustration.
Incidentally, in the wafer polishing apparatus adapted for the polishing technique mentioned above, commercially available polishing cloth is generally used in its unmodified form as a polishing member to be disposed on a polishing table. The polishing cloth is known in the two types, namely the suede type and the velour type. These two types are selectively used to suit the purpose of polishing.
The suede type polishing cloth is a man-made leather for the industrial application so to speak. It is composed of a substrate layer of three-dimensionally constructed non-wovenfabric formed of synthetic fibers and a special synthetic rubber and a surface layer having numerous minute pores fonned in such resin as polyurethane excelling in abrasion resistance. The velour type polishing cloth is a so-called monolayer non-woven fabric, namely a three-dimensionally constructed porous sheetlike material.
For the polishing of a wafer is adopted a method which comprises pressing a wafer held fast with a retaining member under prescribed pressure against an polishing cloth fixed on the polishing table and polishing the wafer while feeding a suitable polishing agent onto the polishing cloth.
The polishing cloth which is used for primary polishing and secondary polishing of a wafer is constructed in such a hard texture as minimizes the possible dispersion of wall thickness of the polished wafer and is designed to remove by polishing the parts of a large wall thickness preferentially. With the wafer polishing apparatus which is provided with such a polishing cloth as described above, therefore, the surface-based polishing mentioned above is attained only with difficulty.
For the purpose of eliminating this difficulty, a polishing apparatus illustrated in FIG. 13 and a "mirror polishing apparatus for a wafer" disclosed in JP-A-05-69,310 have been proposed, for example.
The polishing apparatus of FIG. 13 comprises a pressing member 71 made of a hard material, a soft mounting pad 72 attached as a wafer retaining plate to the lower surface of the pressing member 71, all annular template 73 disposed on the lower surface of the pad 72, and a soft polishing cloth 75 disposed on the surface of a polishing table 74. The polishing apparatus set forth in JP-A-05-69,310 mentioned above, as illustrated in FIG. 14, comprises a soft elastic film 51 having a plane for retaining a wafer W, an annular barrel part 52 having the elastic film 51 attached thereto with uniform tension, and fluid feed means 53 for feeding a fluid for adjusting the pressure exerted on the wafer W to the surface of the elastic film 51 opposite to the surface thereof holding the wafer W thereon. In the diagram, 54 stands for a rotating shaft, 55 for an annular guide plate (template) attached to the lower surface of the elastic film 51, and 56 for a stationary polishing table.
Incidentally, the amount of the wafer to be removed by polishing depends largely on the polishing pressure. For the surface-based polishing technique mentioned above, therefore, it is extremely important that the wafer is polished so as to uniformize the amount of removal due to polishing throughout the entire surface of the wafer as illustrated in FIG. 15 (b) by uniformizing the distribution D of the polishing pressure exerted on the rear surface of the wafer W (equally distributed load) as illustrated in FIG. 15(a). In FIG. 15(a), 61 stands for a wafer retaining member and 61 for an polishing cloth.
The polishing apparatus illustrated in FIG. 13, in spite of the advantage in simplifying the construction for retention of a wafer, succumbs readily to the influence of dispersion of the characteristic properties (thickness, elasticity, and inclination toward deterioration) and does not easily attain uniformization of polishing pressure. As respects the distribution D of polishing pressure, therefore, the polishing pressure within the wafer surface lacks uniformity as shown in FIG. 16(a) and the polished wafer W produces an undulation A as shown in FIG. 16(b) when the mounting pad to be used has a dispersed thickness, the polished wafer W produces a protrusion B in the peripheral part thereof as shown in FIG. 17(b) when the polishing pressure is unduly small in the outer circumferential part of the wafer as shown in FIG. 17 (a), and the polished wafer W produces a peripheral sag C as shown in FIG. 18(b) when the polishing pressure is unduly large in the outer peripheral part of the wafer as shown in FIG. 18(a).
The polishing apparatus disclosed in JP-A-05-69,310 mentioned above is required to set the distance between the lower surface of the outer edge part of the elastic film 51 and the upper surface of the polishing table 56 accurately within a prescribed range for the purpose of curbing the occurrence of an abnormal shape in the circumferential part of the wafer as shown in FIG. 17(b) and FIG. 18(b) because the elastic film 51 serving to seal the annular barrel part 52 abounds in flexibility.
If this distance is unduly large, the polished wafer W will assume such a cross-sectional shape as shown in FIG. 17(b) because the central part of the elastic film 51 is caused to form a convex surface by the pressure of fluid. If the distance is unduly small, the polished wafer W will be made to assume such a cross-sectional shape as shown in FIG. 18(b) by the load exerted downwardly by the barrel part 52 or the pressure of fluid exerted between the wafer W and the barren part 52. In either case, the oxide film of the wafer cannot retain the uniformity of thickness.
SUMMARY OF THE INVENTION
This invention has been produced with a view to eliminating the drawbacks of prior art mentioned above. It is a primary object of this invention to provide a polishing member and a wafer polishing apparatus which are capable of implementing surface-based polishing without compelling a wafer to produce anundulating surface, a peripheral protrusion, or a peripheral sag.
The first aspect of this invention recites a polishing member disposed on a polishing table, characterized by having a foam sheet of soft rubbery elastomer and a polishing cloth laminated.
The second aspect of this invention recites a polishing member disposed on a polishing table, characterized by having a flexible sheetlike member of a hard thin sheet interposed between a foam sheet of soft rubbery elastomer and a polishing cloth.
The third aspect of this invention recites a polishing member according to the first or second aspect of this invention, characterized in that the foam sheet is a closed-cell foam which is made of natural rubber, synthetic rubber, or thermoplastic elastomer and vested with flexibility by the gas in the cells thereof and the foam sheet has (1) a thickness in the range of from 0.2 to 2 mm, (2) a cell diameter in the range of from 0.05 to 1 mm, (3) a cell content (the ratio of the total volume of cells to the total volume of the foam sheet) in the range of from 70 to 98%, and (4) a compressive elastic modulus in the range of from 10 to 100 g/mm.sup.2.
The fourth aspect of this invention recites a polishing member according to the first or second aspect of this invention, characterized in that the polishing cloth is of the suede type or of the velour type.
The fifth aspect of this invention recites a wafer polishing apparatus, characterized by having a foam sheet of soft rubbery elastomer superposed fast on the surface of a polishing table and having a polishing cloth laminnated on the foam sheet.
The sixth aspect of this invention recites a wafer polishing apparatus, characterized by having a foam sheet of soft rubbery elastomer superposed fast on the surface of a polishing table, having a flexible sheetlike member of hard thin sheet laminated on the foam sheet, and having a polishing cloth laminated on the flexible sheetlike member.
The seventh aspect of this invention recites a wafer polishing apparatus according to the fifth or sixth aspect of this invention, characterized in that the foam sheet is a closed-cell foam which is made of natural rubber, synthetic rubber, or thermoplastic elastomer and vested with flexibility by the gas in the cells thereof and the foam sheet has (1) a thickness in the range of from 0.2 to 2 mm, (2) a cell diameter in the range of from 0.05 to 1 mm, (3) a cell content (the ratio of the total volume of cells to the total volume of the foam sheet) in the range of from 70 to 98%, and (4) a compressive elastic modulus in the range of from 10 to 100 g/mm.sup.2.
The eighth aspect of this invention recites a wafer polishing apparatus according to the fifth or sixth aspect of this invention, characterized in that the polishing cloth is of the suede type or of the velour type.